Tidal forces are due to differential gravity forces on an extended body.  In the figure above the Moon's gravitional force per unit mass (gravitational field) is shown acting at various points on the Earth's surface and at the center of the Earth.  The red arrows show the vector difference between the forces acting at the Earth's surface and the force acting at the center.  Note that these tidal forces "stretch" the Earth towards and away from the Moon and "squeeze" or compress the Earth perpendicular to this direction.

This diagram uses the principles of orbital mechanics to explain how tidal forces cause a "stretching" force on an extended object.

Tidal mechanisms for spin-orbit synchronization.  Earth-Moon example:  1-1 spin-orbit synchronization.